Several aspects of gene structure and function are being studied in cultured cell lines and organ cultures from Drosophila melanogaster. In situ hybridization experiments with polytene chromosomes are being used to analyze populations of cytoplasmic messenger RNA and to compare them with the heterogeneous nuclear RNA from the same cells. Cytoplasmic mRNA from rapidly growing cultured cells hybridizes with many polytene bands, distributed over the chromosome set, as well as with some heterochromatic regions in the chromocenter. The relationship of these chromosomal sites to various classes of mRNA defined in terms of relatve RNA abundance, lifetime, or poly A content is under investigation. When Drosophila cells are subjected to a 37 degrees C heat shock, the newly synthesized mRNA contains only a small number of RNA species compared to those present at 25 degrees C. The RNAs hybridize to regions of the chromosome which undergo puffing when a similar heat shock is applied to intact larvae. Analysis of "heat shock" nuclear RNA shows that heat shock causes a decrease in transcrption from normal sites and an increase (or turn-on) of transcription of normal loci. "Heat shock" mRNA can be resolved into a number of discrete bands by polyacrylamide gels. We are using heat shock as a model system to study gene induction, polytene chromosome structure and the relationship between nuclear and cytoplasmic RNA. BIBLIOGRAPHIC REFERENCES: Spradling, A., Penman, S. and Pardue, M.L. Analysis of Drosophila mRNA by In Situ Hybridization: Sequences Transcribed in Normal and Heat Shocked Cultured Cells. Cell 4: 395-404 (1975). Lengyel, J.A. and Pardue, M.L. Anaysis of hnRNA Made During Heat Shock in Drosophila melanogaster culture cells. Abstract No. 479, annual meeting of the American Society for Cell Biology, 1975.